Quadrupole electrode and process for producing the same

ABSTRACT

The present invention relates to improvement of a quadrupole electrode for use in a mass spectrometer or the like, in which two pairs of electrode rods 1, 2, 3 and 4 formed in such a manner that the section of the opposed face of each rod is hyperbolic or circular, and each electrode rod is made of a ceramic and the surface of the electrode is coated with a coating layer 5 of a conductive metal. Further, the present invention relates to a production process, characterized by incorporating such four electrodes at predetermined intervals. Since the electrodes are mainly made of a ceramic which is easily formable with a high dimensional accuracy, the adjustment of the positional relationship between the electrodes during assembling can be made without much effort, which enables a quadrupole electrode having a high performance to be provided with a good reproducibility at a low cost.

TECHNICAL FIELD

The present invention relates to a quadrupole electrode for use in thesensor part of a mass spectrometer or the like.

BACKGROUND ART

A quadrupole electrode used in a mass spectrometer of the like comprisesfour electrodes 11, 12, 13 and 14 formed in such a manner that opposedsurfaces are hyperbolic in their cross section as shown in FIG. 4, orfour electrodes 11', 12', 13' and 14' formed so as to have a circularcross section as shown in FIG. 5 are disposed in a positionalrelationship adjusted so that the electrodes are located atpredetermined intervals. When ions are fed into the center of thequadrupole electrode in the direction indicated by an arrow, it becomespossible to take out ions having a particular mass to charge ratio witha high accuracy from the opposite side of the quadrupole electrode. Insuch a conventional quadrupole electrode, the distance between theelectrode rods should be kept so accurately that a very highly accuratework is required in assembling the quadrupole electrode and a long timeare necessary for the assembly and adjustment of the quadupoleelectrode. Further, a change in the distance between the electrodescaused during the analysis should be minimized.

For example, Japanese Patent Laid-Open No. 30056/1983 describes the useof an electrode produced by subjecting a metallic material to extrusionor drawing into a V-shaped electrode for the purpose of reducing theweight of the electrode and, at the same time, improving the dimensionalaccuracy. Further, Japanese Patent Laid-Open No. 87743/1984 and JapaneseUtility Model Laid-Open No. 64562/1985 describe the shape of electroderods which are easy to assemble into a quadrupole electrode. Further,other various designs have been proposed in the art.

In the conventional quadrupole electrode, in order to bring the accuracyof the distance between the constituent electrodes to a predeterminedvalue, it is a common practice to use a method which comprises manuallyassembling a quadrupole electrode, introducing a monitor gas forconfirming the accuracy and repeating a check on the accuracy to correctthe distance between the electrodes. According to the present invention,the constituent electrodes can be disposed with a high dimensionalaccuracy without any such troublesome work and the predeterminedaccuracy of the distance between the electrodes can be kept high duringthe use thereof.

The present invention provides a quadrupole electrode comprising twopairs of opposed electrodes, characterized in that the electrode rodsare constituted of electrode rods which are made of an insulatingceramic and coated with a conductive metal, and are previously fixedwith a predetermined dimensional accuracy.

The section of the opposed face of each electrode is a hyperbolic orcircular. The ceramic constituting the electrode rod has a coefficientof thermal expansion of 9(×10⁻⁶ /°C.) or less, more preferably acoefficient of thermal expansion of 4(×10⁻⁶ /°C.) or less.

The present invention provides a process for producing a quadrupoleelectrode which comprises incorporating the above-mentioned fourelectrodes at predetermined intervals in such a manner that two pairs ofthe electrodes are arranged opposite to each other. In the production,the four electrodes are jointed to each other directly or through a jig.

Thus, the present invention has been made with a view to facilitatingthe formation of a quadrupole electrode with a high accuracy and a goodreproducibility. In the present invention, a high accuracy within ±5 μmcan be attained in the distance between the electrodes and a change inthe distance between the electrodes during the use thereof in theanalysis can be minimized by using an insulating ceramic having a lowcoefficient of thermal expansion and subjected to high-accuracy workingas the material of the electrode and, after coating the surface of theelectrode with a conductive metal, assembling four electrodes, andincorporating the resultant quadrupole electrode in a mass spectrometer.

In order to improve the accuracy of assembling a quadrupole electrodeand, at the same time, to shorten the time necessary for the adjustmentof the accuracy, it is necessary to assemble at once the electrodes intoa quadrupole electrode through reference planes finished with apredetermined accuracy. When a metal is used as the material of theelectrode, however, there occurs a problem that the insulation betweenthe electrodes cannot be maintained. This problem can be solved throughthe use of an insulating ceramic. Since ceramic has a low coefficient ofthermal expansion and a light weight, it is advantageous in that thedimensional stability against a change in the temperature can bemaintained and improved and the handleability is good. A ceramic havinga coefficient of thermal expansion of 9(×10⁻⁶ /°C.) or less suffices forthis purpose, and use may be made of Si₃ N₄, sialon, mullire, SiC, AlN,Al₂ O₃, cordierire, quartz, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of one embodiment of the presentinvention.

FIG. 2 is a graph showing the results of measurements of scattering ofthe peak waveforms in a mass spectra given by a mass spectrometer.

FIG. 3 is an explanatory view of an embodiment wherein the electrode ofthe present invention is incorporated in a mass spectrometer.

FIG. 4 is an explanatory perspective view of one construction of theconventional quadrupole electrode.

FIG. 5 is an explanatory perspective view of another construction of theconventional quadrupole electrode.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will now be described in more detail with reference toFIG. 1. Numerals 1, 2, 3 and 4 designate four electrodes previouslysubjected to high-accuracy working, and the body of each electrode rodis made of a ceramic. Although the ceramic may be any one as far as ithas an insulating property and a low coefficient of thermal expansion,it is particularly important that the coefficient of thermal expansionbe small. The present inventors have made intensive studies through theuse of various ceramics and, as a result, have found that a coefficientof thermal expansion of 9(×10⁻⁶ /°C.) or less suffices for this purposeand Al₂ O₃, SiC, mullire, quartz, sialon, AlN, cordierire and Si₃ N₄ areeffective. As a result of further detailed studies on these ceramics, ithas been found that an Si₃ N₄ ceramic having a coefficient of thermalexpansion of 4(×10⁻⁶ /°C.) or less is preferred. This is because thedistance between the electrodes of the quadrupole electrode of a massspectrometer where a high resolution is required is as large as at least20 mm and, in this case, a change in the distance between the electrodeswith the elapse of time is believed to affect the accuracy of analysis.

The use of a Si₃ N₄ ceramic electrode having a low coefficient ofthermal expansion enables the distance between the electrodes to be keptwith an accuracy as high as ±5 μm, that is, the analytical accuracy tobe sufficiently maintained, even when use is made of a quadrupoleelectrode having a large distance between the electrodes.

Numeral 5 designates a conductive metal layer formed for coating thesurface of the ceramic therewith for the purpose of allowing the ceramicto function as an electrode. The formation of the metal layer enablesthe insulating ceramic to function as the electrode. The metal layer maycomprise any conductive metal, and it is also possible to use a singlephase composed of Mo, W, Au, Pt, Ti, Cu, Ag, Ni or the like or an alloyor a composite phase composed of these materials. The thickness ispreferably 1 mm or less. When the thickness exceeds 1 mm, there is apossibility that peeling occurs unfavorably. The coating may beconducted through the formation of a thin film according to a vapordeposition process or coating according to the wet paste method. Ifnecessary, the metallized layer may be machined to maintain theaccuracy.

An electrode terminal can be formed by passing a conductive lead wirethrough a hole 7 of each of the electrode rods 1, 2, 3 and 4 forconduction to a conductive metal layer formed on the hyperbolic surfaceof the ceramic electrode rod. The lead wire is fixed with a nut 8. Thus,four ceramic electrodes are formed independently of each other. Theseelectrodes can be assembled with a high accuracy by fixing referenceplanes 1', 2', 3' and 4' of the electrodes to each other by lapping andjointing the electrodes to each other directly or through a jig 6 suchas a chip. The jointing is conducted through the use of an active metallayer for a ceramic, fine particles of a ceramic, or the like.

Thus, it has become possible to facilitate assembling of four ceramicelectrodes each made of a ceramic coated with a conductive metal into aquadrupole electrode with a high accuracy. In the drawing, numeral 9designates a lead wire.

EXAMPLE 1

An electrode body having a distance between the opposed electrodes of8.6 mm and a length of 200 mm was made of an Si₃ N₄ ceramic materialhaving a coefficient of thermal expansion of 3.2×10⁻⁶ /°C. as a ceramicmaterial, and the hyperbolic face thereof was machined with a highaccuracy. Thereafter, an active metal (Ti-Cu-Ag) was deposited thereonin a thickness of 5 μm, and Ni was further deposited thereon in athickness of 1 μm to form electrodes. These electrodes were assembledinto a quadrupole electrode as shown in FIG. 1. As shown in FIG. 3, anion source 16 for forming ions was mounted on one end of the quadrupoleelectrode 15, while a secondary electron multiplier 17 for detectingions was mounted on the other end thereof. Numerals 18 and 19 designatean oscilloscope and a pen recorder, respectively. This assembly wasincorporated as a quadrupole mass spectrometer in an ultrahigh vacuumapparatus where it was baked at 300° C. Thereafter, He, N₂, Ar, Kr andXe gases were flowed, and this procedure was repeated several times tomeasure a scattering in the peak waveform of a mass spectrum. FIG. 2shows the measurement results in which numbers, i.e., 0, 1, 2, 3, 4 and10, are the numbers of baking runs.

As a result, the peak waveform of the quadrupole mass spectrometer, inwhich a conventional metal electrode (Mo electrode) was used, was in thesplit parabolic form as shown in FIG. 2(b). Also, the scattering of thepeak height was large. This scattering of the peak waveform is believedto be attributable to the scattering of the dimensional accuracy. On thecontrary, the peak waveform of the quadrupole mass spectrometer, inwhich the Si₃ N₄ ceramic quadrupole electrode was used, was in theparabolic form as shown in FIG. 2(a), and scarcely any scattering of thepeak height was observed. Thus, the use of the Si₃ N₄ ceramic quadrupoleelectrode has made it possible to simplify the assembling and adjustmentof the electrode and maintain a high analytical accuracy.

EXAMPLE 2

Si₃ N₄ ceramic electrode rods for forming a quadrupole electrode havinga distance between the electrode rods of 8.6 mm and a length of 200 mmwas machined into a predetermined shape having a predetermineddimension, which was then subjected to finish working so that thesection became hyperbolic.

The hyperbolic part was coated with Ti, Cu, Ag and Ni each in athickness of 1 μm by ion plating to form a conductive film having athickness of 4 μm in total. A Kovar rod of 1.6φ was inserted into a holepreviously formed in each electrode and then the electrodes were joinedand fixed by means of an active metal solder.

The four Si₃ N₄ ceramic electrodes were fixed one to another with thereference planes thereof abutting against each other and soldered toeach other with an active metal solder via Si₃ N₄ chips (jigs, 6), 5×5in area and 10 mm long, in a jointing furnace under the conditions of800° C. and 10 min.

The time taken for the assembling was 10 hr, and the accuracy of thedistance between the electrodes in the assembling was within ±5 μm,which enabled the assembling time to be remarkably reduced. Thequadrupole electrode thus assembled was incorporated in a vacuumapparatus, where baking was repeated ten times at 300° C. Then, thescattering of the peak waveform in a mass spectrum was measured. It wasfound that the waveform was parabolic as shown in FIG. 2(a) and noscattering of the peak height was observed. On the contrary, the peakwaveform given by the conventional metal (Mo) quadrupole electrode wasin the split parabolic form as shown in FIG. 2 (b) and the scattering ofthe peak height was significant.

INDUSTRIAL APPLICABILITY

In the present invention, since each electrode rod is mainly made of aceramic which is easily shaped with a high dimensional accuracy, theadjustment of the positional relationship between the electrodes duringassembling can be made without much effort, which enables a quadrupoleelectrode having a high performance to be provided with a goodreproducibility. Further, since a ceramic is used as the main material,it is possible to provide a quadrupole electrode having a light weightat a low cost as opposed to a quadrupole electrode wherein Mo orstainless steel is used as the main material.

We claim:
 1. A quadrupole electrode comprising two pairs of opposed electrodes, each electrode being formed of an insulating ceramic and comprising an inner surface which faces an inner surface of the opposing electrode, and mating surfaces formed to mate with corresponding mating surfaces of adjoining electrodes, a portion of said inner surface being coated with a conductive metal whereby said portion of said inner surface does not contact said portion of said adjoining electrodes,said mating surfaces being formed whereby, when said quadrupole electrode is assembled, each said coated portion will be maintained a predetermined distance from an opposing coated portion.
 2. The electrode of claim 1 wherein said inner surface is hyperbolic.
 3. The electrode of claim 1 wherein said inner surface is circular.
 4. A quadrupole electrode according to claim 1, wherein the ceramic constituting said electrode rod has a coefficient of thermal expansion of 9(×10⁻⁶ /°C.) or less.
 5. A quadrupole electrode according to claim 1, wherein the ceramic constituting said electrode rod is an Si₃ N₄ ceramic having a coefficient of thermal expansion of 4(×10⁻⁶ /°C.) or less.
 6. A process for producing a quadrupole electrode comprising incorporating two pairs of opposed electrodes, each electrode being formed of an insulating ceramic and comprising an inner surface which faces an inner surface of the opposing electrode, and mating surfaces formed to mate with corresponding mating surfaces of adjoining electrodes, a portion of said inner surface being coated with a conductive metal, whereby said portion of said inner surface does not contact said portion of said adjoining electrodes,said mating surfaces being formed whereby, when said quadrupole electrode is assembled, each said coated portion will be maintained a predetermined distance from an opposing coated portion.
 7. The process of claim 6 wherein said inner surface of each electrode is hyperbolic.
 8. The process of claim 6 wherein the inner surface of each electrode is circular.
 9. The process of claim 6 wherein said electrodes are joined to each other directly.
 10. The process of claim 6 wherein said electrodes are joined to each other through a jig. 